An example of an electric power distribution system mounted in an automobile is one that houses an electrical connecting box in a housing shaped cover. The electrical connection box has a structure in which a circuit assembly is accommodated in a case, and the circuit assembly is structured to include a circuit board, bus bars that are wired along the reverse face of the circuit board, and switching members such as relays and the like that are mounted on the obverse side of the circuit board. The case is structured to include a frame shaped member that is disposed and secured along the outside perimeter of the circuit board, a cover that is mounted so as to cover the circuit board from the obverse side in relation to the frame, and a heat sink that is affixed to the reverse face of the circuit board and that covers the opening on the reverse side of the frame.
Note that an example of such an electrical connection box is disclosed in Japanese Patent Application Publication No. JP-A-2003-164039.
In this type of electric power distribution system, parts accommodation portions that are formed on the top face of the frame shaped member are formed so as to be exposed from the opening portion of the top face of the cover, and thereby, attachment and detachment of parts (for example, fuses) in the parts accommodation portions is simplified.
However, in a structure in which the parts accommodation portions are exposed in the opening portion of the cover, there is a concern that water remaining on the top face of the cover due to condensation and the like may penetrate inside the cover through gaps between the opening portion and the parts accommodation portions, and may thereby adhere to the outside face (the top face of the frame shaped member) of the electrical connection box. In this case, there is a concern that water will penetrate inside the electrical connection box through the gaps between the parts that form the case, and as a result, short-circuits and the like will occur in the circuit assembly.